OS .91 SZH Pumped or New YS .91 SR?

[BLUETHUNDER]

I see a lot of OS .91 SZH engines for sale in the clasifieds :?
Are people having trouble with the OS SZH engine or do
they just want to try the new kid on the block the YS .91 SR
When will the new Hatori SB 19 LS be available for the YS .91 SR?

I am looking for a new .91 engine for an EVO 90 kit
What do you recomend and please explain why. I
currently have a couple OS .50 Hyper's in my EVO 50's
and the non pumped OS .91 SZH looks just like a bigger
Hyper 50, does it run just like a hyper 50?

Thanks,

Bob

[Kinger]

Bob,

I've got the non-pumped OS91SZ in my R90SE. Tried the cline setup, but unfortunately it didn't end up working out for me. (Had a problem with the setup going rich inverted for some reason.)

Anyway, the 91SZ in non-pumped form runs VERY much like the OS50 Hyper with the exception being that you can tune the midrange via a 3rd needle assembly. The only downside to the 91SZ in non-pumped form is obviously the leaning you get at the end of a tank of fuel. Kinda annoying at times, but certainly liveable and there are no hassles with a pump or a pressurized fuel tank to deal with.

The other nice thing about the 91SZ is the parts availability. With the old YS motors (91ST4) parts and motors were hard to come by at times. Not sure if that will be the same with the 91SR.

Good luck with whatever you end up with as I'm sure either motor will fit the bill nicely. Oh, one other thing you might want to consider if you are using either motor in the Evo is going to the 7.92 gear ratio instead of the 8:45.

Mike

[BLUETHUNDER]

Mike,

thanks for the info, you make some very good points.
I like the way my hyper 50's run, I can deal with a
little lean run at the end of a tank as I'm usually just
hovering at that point anyway. Great Planes customer
service is the best! YS is hit or miss & you never know
who the distributer is going to be the following year
and if you can get parts etc...

I thought the 1/7.92 gear ratio was for YS engines?
The Raptor 90 is 1 / 8.29 I believe? What is the
advantage of 1 / 7.92 If I was looking for 1900 head
speed in Idle up would 1 / 8.45 work OK as it is close to
the Raptor's 1/ 8.29 and puts the motor in the correct
powerband for use with Hartori's SB - 18FH muffler

Thanks,

Bob

[Kinger]

Hey Bob,

You can use 8:45 with the OS and I know many have posted that they do. I know Havoc here on HF has the Evo with an OS motor. You might want to shoot him a PM to see if he's tried the different gear ratios on the motor and which he prefers.

Mike

[DavidH]

The OS 91 SZ seems to like to be in the mid 15K rpm range.
On 8.18 that is a head speed on 1900

8.45 would yield a max head speed of 1830 for the engine to be at 15,500 rpm.

Don't have any idea what the YS 91 SR Long Stroke rpm range it likes to run. I only know a couple of pilots that have that motor. I believe they are using 7.75 or 7.9 gear ratio.

David